C. trachomatis is the most frequently reported bacterial sexually transmitted disease in the US. In 2008, 1,210,523 chlamydial infections were reported to CDC (http://www.cdc.gov/std/stats08/trends.htm). If untreated, 10% of C. trachomatis-infected women may develop complications such as pelvic inflammatory diseases, ectopic pregnancy and infertility. The annual cost of treating C. trachomatis complications in women is >$2 billion. Due to lack of obvious symptoms after an acute infection, most infected individuals don't seek treatment, thus permitting the development of complications. One solution to this challenge is rapid diagnostics so that antibiotics can be taken to reduce complications. The current diagnosis of chlamydial infection requires specialized labs and takes days for health care providers or patients to obtain results. A second (or long- term) solution is vaccination so that exposure to C. trachomatis no longer causes complications. The failure of Chlamydia organism-based vaccines more than 50 years ago and immunological studies in the past 50 years have led to the conclusion that a subunit vaccine is both necessary and feasible for preventing Chlamydia diseases. Yet, no licensed C. trachomatis vaccine is available. The first identified C. trachomatis-secreted protein (CtSP), known as CPAF [chlamydial protease/proteasome-like activity factor], is a unique serine protease that selectively targets host proteins for evading host defense. Our success in identifying and characterizing CPAF has not only significantly advanced our knowledge on chlamydial pathogenesis but also provided important information for improving chlamydial diagnosis, treatment and prevention of chlamydial infection, which has encouraged us to further identify and characterize additional C. trachomatis-secreted proteins (CtSPs) as proposed in the current grant so that novel molecular mechanisms of chlamydial pathogenesis can be learnt and new knowledge can be obtained to further improve medicine. The three specific aims proposed in the current grant are: Aim I to identify C. trachomatis-secreted proteins (CtSPs) during intracellular infection using a genome-wide antibody localization of endogenous proteins approach; Aim II to characterize 4 sec-CtSPs (CPAF, cHtrA, CT311 & CT795) by mapping their secretion pathways and Aim III to further explore the functions and applications of 2 Sec-CtSPs (CPAF & cHtrA).